It is well known that hot dip galvanized steel strips have a superior anti-corrosion property and, therefore, are used as anti-corrosion materials in various fields, for example, buildings, construction, home appliances, automobile bodies, etc. In Japan, this type of zinc-coated steel strip is produced in an amount of six million metric tons per year, which corresponds to about 36% of the entire yearly production of cold-rolled steel strips.
Zinc is an inexpensive metal having a proper chemical activity. Also, zinc can be converted to a zinc compound having a dense structure which causes the corrosion of the zinc compound to be retarded. Therefore, zinc is a metal highly effective for protecting steel strips from corrosion. The corrosion rate of zinc is variable depending on the type and properties of the corrosion product and the type of the corrosional environment. For example, when zinc corrodes in an atmosphere containing a large amount of sulfurous acid gas or in an acid or alkaline environment, the resultant zinc compound is soluble in a sulfurous acid gas solution or an acid or alkali solution, respectively. Therefore, in the above-mentioned atmosphere or environment, the zinc coating layer cannot satisfactorily protect the steel strips.
However, when the zinc is coated in a neutral environment, the resultant zinc compound exhibits a dense structure and is insoluble in a neutral solution. Therefore, in this environment the zinc coating can satisfactorily protect the steel strips except in the following case. In the case where the neutral environment contains a large concentration of chlorine ions, for example, a road on which sodium chloride is sprinkled, zinc is corroded at a relatively high rate. Therefore, for example, the automobile body made of a zinc-coated steel sheet does not exhibit a satisfactory resistance to corrosion on a road on which salt is sprinkled.
In a conventional zinc-coated steel strip placed in a neutral environment, the self-sacrifice anti-corrosion activity of the zinc coating to the steel strip substrate is excessive in comparison with the necessary smallest activity thereof. Therefore, even if the corrosion rate of the zinc coating is decreased, the zinc coating can exhibit a satisfactory self-sacrifice anti-corrosion activity. For example, even if the corrosion rate of pure zinc placed in a 3% sodium chloride aqueous solution is decreased to a level of 1/20 to 1/50 of the original rate thereof, pure zinc can exhibit a satisfactory self-sacrifice anti-corrosion effect on the steel strip. Accordingly, in the above-mentioned environment, if the corrosion rate of the zinc coating is decreased to the level of 1/20 to 1/50 of the original rate, the zinc coating can exhibit an enhanced durability of 20 to 50 times the original durability of the zinc coating. Also, in order to keep the same level of durability as the original durability, the weight of the zinc coating can be reduced to a level of 1/20 to 1/50 of the original weight of the zinc coating.
In the past, zinc-coated steel strips were mainly used in buildings and construction. However, at present, the use of zinc-coated steel strips has spread to home appliances, automobiles and steel-made furniture. Therefore, zinc-coated steel strips are required to exhibit properties suitable for the above-mentioned new uses. That is, the zinc-coated steel strips should exhibit, in addition to an excellent resistance to corrosion of the steel strips,
(1) an enhanced adhesion between the steel strip and the zinc coating, PA1 (2) a satisfactory appearance and no color-change, and PA1 (3) an enhanced finish coating property on the zinc coating. PA1 coating at least one surface of a steel strip with a melt of a zinc-based alloy; PA1 controlling the weight of the melted zinc-based alloy coating formed on said steel strip surface, and; PA1 solidifying said melted zinc-based alloy coating,
The finish coating may include a chemical conversion treatment, Organic Coating and paint coating.